The long range objectives of this project is to clarify cardioinhibition during cardioregulation in Limulus polyphemus. Gamma-aminobutyric acid (GABA) and 5-hydroxytryptamine (5-HT) both have been implicated as cardioinhibitory transmitters within Limulus polyphemus Central Nervous System (CNS). The precise physiological role played by GABA in cardioinhibition is not settled. In our effort to clarify neural control of inhibition in the CNS of Limulus polyphemus, we hypothesize that GABA co-exists with 5-HT in the CNS of Limulus as a neurotransmitter, and that GABA aids in a inhibitory response during cardioregulation in both the central and peripheral nervous system. We propose to identify GABAergic neurons within the CNS of Limulus by using immunocytochemistry. The identification of immunoreactive somata and fibers to immunoreactive somata. The antagonistic action of picrotoxin, and bicuculline will be assessed electrophysiologically, on GABA inhibitory response during analysis of burst frequency, burst content (spikes/burst), and burst duration. We propose to further characterize pharmacologically GABAergic receptors within Limulus CNS with the use of agonists and antagonists. Kinetics and pharmacological studies on the uptake and release of GABA is putative inhibitory neurotransmitter in Limulus and function in cardioregulation both in the central and peripheral nervous system.